1. Field of the Invention
The present invention relates to a phase shift mask to be used in forming a pattern for use in a lithography process in the fabrication of a semiconductor device, and more particularly to a phase shift mask capable of preventing an undesirable pattern formation at an edge portion thereof where it is of the positive type and a method for fabricating such a phase shift mask.
2. Description of the Prior Art
As semiconductor devices have a higher degree of integration, a more sophisticated photo lithography technique is needed. In this connection, there has been much research into making a phase shift mask with a phase shift material formed on a transparent mask substrate. However, such a phase shift mask involves various problems solved because it has difficulties in design, fabrication and defect control.
FIG. 1 is a plan view illustrating a conventional positive type phase shift mask. FIG. 2 is a diagram explaining the concept of a phase shift at a pattern edge. On the other hand, FIG. 3 is a plan view illustrating a pattern formed using the mask shown in FIG. 1. The conventional phase shift mask and problems encountered in using the conventional phase shift mask will be described in conjunction with FIGS. I to 3.
The mask shown in FIG. 1 is called a "Levenson type mask" which is a phase shift mask exhibiting a superior processing effect. Furthermore, this mask can be fabricated using a positive photoresist film because it is designed as being of the positive type. However, this mask involves generation of a reverse phase shift of light at a region 5 where the edge of a phase shift material is in contact with a substrate, as shown in FIG. 3. This results in formation of an undesirable pattern 4, as shown in FIG. 3.
On the other hand, in case of a negative type mask, the above-mentioned problem encountered in the positive type mask is not generated. However, the entire process needs to be varied to be compatible with a negative photoresist film because all of the existing processes are designed to be compatible only with the positive photoresist film.